1. Field of the Invention
The present invention relates generally to spark plugs, and more particularly to spark plugs having an extended shell and insulator.
2. Related Art
Spark plugs have been used for many years to provide a means to ignite the fuel air mixture in the combustion chambers of an internal combustion engine. Spark plugs have taken on many forms to adapt to the particular engine design and environment. Generally, spark plugs have a center electrode surrounded by an insulator wherein the insulator is disposed in and captured by a metal housing or shell. The shell typically has a plurality of threads which are matched to the bore threads in the engine block. The threads allow the spark plug to be screwed into the bore using a conventional tool. Further, the shell includes a ground electrode extending from an end of the shell proximate the center electrode. The ground electrode together with the center electrode define a spark gap. The shell also acts as a ground shield to provide an electrical ground path from the spark gap to the engine block.
The spark plug seats or seals against the engine cylinder head to seal the combustion chamber and prevent combustion gases from escaping through the spark plug hole in the cylinder head. Commonly, the seat is located above the threads and is combined with a sealing gasket that has an interference fit with respect to the threads so as to retain the gasket during installation of the sparkplug.
Increasingly, engine designs employing multiple valves, fuel injection points, coil on plug ignition systems, combustion related sensors and other features have placed increasing demands on the space in the cylinder head immediately adjacent to the combustion chamber, particularly the space above the combustion chamber, which have in turn made it desirable to minimize the space envelope needed for the spark plug, particularly in the lower portions of the spark plug proximate the spark gap where the spark plug is exposed to the combustion chamber and combustion gases.
In addition to restrictions on the space envelope available for the spark plug on the sparking end, in applications where space is restricted, there is also a trend toward higher engine operating temperatures which increases the temperatures to which the spark plugs operating in this restricted space envelope are exposed, making it desirable to improve the ability of the spark plug to remove the heat resulting from operation of the spark plug and the associated combustion processes (i.e., the need for colder spark plugs).
Another common requirement for spark plugs is that they be able to operate without replacement for extended periods of engine and vehicle operation, such as 50,000 or even 100,000 miles of operation.
These space restrictions have led to the use of spark plugs having smaller diameters (e.g., 12 mm, 10 mm and smaller) to achieve the necessary space envelope and heat removal properties, but the manufacture of smaller diameter spark plugs presents other challenges associated with the performance and manufacture of the various spark plug components, such as the insulators and electrode materials.
Another approach has been to extend the spark plug shell maintaining a larger upper portion (e.g., 16 mm), since there is frequently still space available in the head away from the combustion chamber to receive the larger diameter, while reducing the diameter and extending the shell to reach the combustion chamber so as to meet the restricted space envelope requirements. One such spark plug configuration is described in U.S. Pat. No. 5,918,571 to Below which describes an extended shell spark plug where the shell is of two-piece construction of a retainer for the insulator and a ground shield. Below describes the construction by teaching that the insulator and its included center electrode are axially passed into the cylindrical shell ground shield. The flared frustoconical flange of the ground shield engages the insulator shoulder and the cylindrical shell retainer is then passed over the insulator from the opposite end and its interior frustoconical ledge engages a second shoulder of the insulator. A portion of the retainer is then radially collapsed about the flange to secure the ground shield and retainer together with the insulator captured therebetween. The formed portion also serves as the seat for the spark plug. While Below is not specific as to the material of construction, commercial products having the configuration and construction of Below have been observed to utilize a steel retainer and a higher temperature alloy for the ground shield, such as Inconel 600. The two-piece construction has attendant reliability concerns associated concerns when using standard reliability analysis such as Failure Modes Effects Analysis (FMEA) associated with the presence of the additional mechanical compression joint in the spark plug, which has an associated probability of failure. Further, it is believed that placement of the spark plug seat on a formed part which is subject to manufacturing variances associated with two parts may provide an attendant variability of the seat that has a possibility to affect the performance of the seat and the spark plug, as well as the performance of the engine in which it is installed.
While such prior art spark plug designs having extended shells and insulators have achieved their intended purposes. Therefore, a need exists to for spark plugs configured meet the space envelope restrictions while effectively dissipating excessive heat and durable enough to withstand the harsh environments of an internal combustion engine.